You are right

TLT is risky (20 yrs). SHY is safe at 1-3 yrs. But if economy suddenly picks up, SHY will go down. If Fed raises interest rate within the next year, SHY will also go down.
If you can sense the pulse of the economy, you can always un-park the money from SHY.

Personally, I use SHY for parking cash, and use TIP and GLD to hedge against potential inflation as a result of the huge liquidity in the market.